Plansifter mounting and clamping apparatus

ABSTRACT

A plansifter having a vibratory mounting and at least two stacks of sieves formed by a number of sieve frames mounted one upon the other, the said stacks of sieves being disposed in symmetrical positions about a vibrator and comprising stack clamping devices which produce vertical forces.

United States Patent [191 Keller PLANSIFIER MOUNTING AND CLAMPING APPARATUS [75] Inventor: Alois Keller, Oberuzuil, Switzerland [73] Assignee: Gebrueder Buehler AG, Uzwil,

Switzerland [22] Filed: Jan. 16, 1973 [21] Appl No.: 324,060

[30] Foreign Application Priority Data Jan. 20, 1972 Switzerland 866/72 [52] US. Cl 209/319, 209/332, 209/405 [51] Int. Cl B07b 1/28 [58] Field of Search 209/319, 332, 405, 311, 209/414 [56] References Cited UNITED STATES PATENTS Neal 209/311 X June 11, 1974 2,500,966 3/1950 Thompson 209/332 2,668,621 2/1954 Thompson 209/319 2,723,75l 11/1955 Conkling ..209/3l9 Primary Examiner-Frank T. Yost Attorney, Agent, or Firm-Merchant, Gould, Smith & Edell 57 ABSTRACT A plansifter having a vibratory mounting and at least two stacks of sieves formed by a number of sieve frames mounted one upon the other, the said stacks of sieves being disposed in symmetrical positions about a vibrator and comprising stack clamping devices which produce vertical forces.

11 Claims, 9 Drawing Figures SHEEI 10F 4 PATENTED JUHI l 1974 PLANSIFTER MOUNTING AND CLAMPING APPARATUS BACKGROUND AND SUMMARY OF THE INVENTION In a known plansifter of this kind, two open stacks of sieves are disposed on a horizontal platform symmetrically about the vertical axis of a rotating unbalanced weight. The wheel-form unbalanced weight is disposed in the form of a horizontal disc directly below the platform. The whole unit bears centrally on a base through four vertical bearings. The two stacks of sieves are each directly clamped to the platform by means of an upper retaining frame through a number of tie rods. In this construction, there are no housing parts. In this way, the stacks of sieves can be visually observed from all sides, and after slackening of the clamping device easy and rapid access to the sieve frames is obtainednThis construction is very suitable for small sifting performances, above all where frequent changes of product are involved, as in the chemical industry.

However, since ever-increasing outputs of a like product are required even in these branches of industry, the absence of a suitable, open large-scale plansifter is sometimes felt.

In the majority of milling plants, large-scale plansifters of closed construction are now employed.

In a known high-performance plansifter of this kind, a solid double frame is disposed around the whole plansifter in different vertical planes. This rigidly constructed frame directly takes up the vibration-exciting forces and transmits them to the sieve compartments, and along the reverse path the reaction forces. These plansifters designed with vertical longitudinal frames permit a very large number of sieve compartments which form a row with the drive compartment. The doors of the compartments are all aligned in the longitudinal direction. However, the rotating unbalanced mass produces vibratory forces not only in the longitudinal direction, but also in the transverse direction thereto, as seen in a horizontal plane. This is the force of a vector rotating about a vertical axis. The service doors are therefore constructed as stay elements to take up the horizontal mass forces of the sieve frames.

A further plansifter of closed construction comprises two important horizontally disposed frame parts, which are disposed substantially midway of the height of two opposite sides. The other two sides are free and have service doors over their entire area.

This plansifter is built up on the unit assembly principle of three housing units, i.e. two sieve boxes and one drive box and may be assembled, for example, by the customer. These three housing units are held together by the horizontal frame elements. The service side of the door comprises no horizontal frame element. However, in order that the forces arising in. the system may be controlled, the doors themselves have been utilized, with the aid of a special door fastening, as a horizontal link between the lateral horizontal frame elements, so that a closed horizontal frame is obtained in the operative condition.

The first-mentioned plansifter which has two open sieve stacks permits only a limited number of sieve frames, because considerable wobbling movements will occur with too many sieve frames disposed one above the other. However, the great advantage of such open plansifters resides in the possibility of a continuous 'visual observation and rapid access to the sieve frames.

On the other hand, closed plansifters permit a very large number of sieve frames disposed vertically one above the other, and also generally a large number of stacks formed by the sieve frames. In the closed construction, vertical walls and doors are often designed with additional channels and permit a very large number of different run-through schemes without any great additional cost.

The invention has for its object to construct a convenient plansifter which combines substantially the advantages of the known open and closed plansifters.

The plansifter according to the invention is characterized in that the forces of the vibration generator are passed into top and bottom elements of two upright stack frames which are rigidly closed in themselves in the transverse direction and embrace as a unit at least one sieve stack.

These surprisingly simple features according to the invention afford a stable operating behavior even with high, open sieve stacks. The transverse arrangement of the closed frame renders possible rapid and convenient access to the sieve frames, which are securely held and nevertheless permit continuous visual observation from at least three sides. The vibratory forces are directed by the drive into the stack frames. The transverse frame forms with the clamping device which produces vertical forces and the sieve stack enclosed by the transverse frame or frames a completely self-contained unit in regard to the forces set up. There is obtained with lower expenditure of material a vibrationally stable, open plansifter without any additional enclosure or housing parts, even for a large number of sieve frames disposed vertically one above the other.

Despite its simplicity, the subject of the invention differs from further, earlier constructions having freely visible sieve frames which might subsequently be assumed to constitute a solution to the same, but novel problem.

In an earlier plansifter of this kind, a number of stacks of sieves are disposed symmetrically about a vertical axis of rotation of a vibrator. The vibratory forces are transmitted substantially through an upper and a lower main beamto the sieve stacks. These upper and lower main beams extend over the whole plansifter, i.e. over the sieve stacks and over the clearance provided for the vibrator between the stacks. The vertical forces produced by a clamping device for clamping the individual sieves in position are taken up at the ends of the continuous longitudinal beams by vertical distance pieces, which are disposed simultaneously on both sides of the sieve stacks and serve to guide the sieve boxes. A disadvantage of this design is the cumbersome construction, which is due above all to the fact that a frame construction embracing the whole plansifter is chosen. Hence, difficulties arise in manufacture and in assembly, because large-scale plansifters have a large base area if one imagines them as being put into an existing room. Therefore, this construction could only be preferred today in the case of small plansifters.

A further disadvantage of this design arises from the fact that the principle at least of a closed frame is not followed. Consequently, the whole plansifter loses a great deal of its stability, which is a further determent to constructing large plansifters in accordance with this principle.

In a further earlier plansifter, there is therefore provided an actual, horizontally disposed longitudinal frame which surrounds the whole system. In this construction, the resultant of the rotating unbalanced masses is positively directed in a horizontal plane which is formed by the longitudinal frame and which contains the center of gravity of the sieve stack, and the flow of forces and the bracing are satisfactorily dealt with. In practice, however, this solution does not constitute any advantage. The reason for this resides primarily in the sub-division into an upper sieve stack and a lower sieve stack, which is awkward for the operating personnel. More particularly where products are frequently changed, as in the chemical and allied industries, rapid and above all convenient access to the sieve frames is extremely important. Therefore, this horizontally closed longitudinal frame is disadvantageous from the operational viewpoint.

In a further known construction, a number of sieve stacks are disposed symmetrically about the axis of the rotating unbalanced mass and held by a bottom frame and a top frame. The plansifter is mounted in freely oscillating manner by means of four rods disposed at the corners. The attachment between the rods and the plansifter is carried approximately midway of the height by four rods which connect the bottom and top frames. The great disadvantage of this construction resides in the absence of any uniform concept for the flow of the forces, with the attendant disadvantages.

The earlier open plansifters therefore cannot achieve the underlying object of the invention.

In a particularly advantageous form of construction of the plansifter according to the invention, the top and bottom elements of the stack frames are each connected through a vibrationally stable top and bottom intermediate member (22, 23) respectively, and the vertical elements of the stack frames form the vertical edges (corners) of a cube-like vibrating frame. A surprising feature of this solution according to the invention is the ideal form of a cube-like frame which is thereby obtained. Nevertheless, the unit assembly type of construction is retained. Each of the two stack frames with the associated sieve stacks forms a transportable and mountable unit, while the space defined by the top and bottom intermediate members provides, with small auxiliary struts at the four corners and with the whole installed drive, a transportable unit.

In a further advantageous form of construction of the plansifter according to the invention, the vibrationexciting forces are directed through punctiform connecting points immediately into the top and bottom elements of the stack frame. Since alternating blending loads can be reduced to a minimum in this embodiment, the useful life is very high, even with a small expenditure for material.

The stack frames comprise a double clamping device for each sieve stack. The forces produced by this clamping device are intended to press the sieve frames together in a vertical direction and to transmit the mass forces by frictional engagement through the individual sieve frames to the bottom and top elements of the stack frames.

In a further form of construction of the plansifter according to the invention, the end position of the clamping device in its clamping condition is situated close to a central plane, containing the vertical elements of the stack frames. of the sieve stacks. The clamping force can thus be taken directly in the form of a counter-pull by the corresponding vertical element of the rigid stack frame.

In a further form of construction of the plansifter according to the invention, there are disposed at the bottom and at the top of the transverse frame angular safety elements, by the closing force of which the sieve stack firmly compressed in a vertical direction by the clamping device is pressed horizontally into a V-shaped back portion.

In a further advantageous form of construction of the plansifter according to the invention, the vibrationexciting forces are produced by an eccentric shaft rotating about a vertical axis.

For a better explanation of the plansifter according to the invention, reference will be made to the figures:

FIG. 1 is an elevational view, partly in section through the drive, of the plansifter according to the invention;

FIG. 2 is a side elevation corresponding to FIG. 1;

FIG. 3 is a plan view corresponding to FIG. 1;

FIG. 4 illustrates the vertical clamping device for the sieve stacks in section along the line IVIV of FIG. 1;

FIG. 5 illustrates the vertical clamping device in section along the line V-V of FIG. 4;

FIG. 6 illustrates the horizontal sieve securing means in section along the line VI-VI of FIG. 1;

FIG. 7 illustrates the lug-form construction of the sieve frames in vertical section along the line VII-VII of FIG. 2; 1

FIG. 8 illustrates in perspective the introduction an removal of the sieve frames; and

FIG. 9 is a view in perspective of the plansifter according to the invention in skeleton form, wherein (A) is a vertical longitudinal plane, (B) is a horizontal plane, and (C) is a vertical transverse plane.

The plansifter illustrated in FIGS. 1, 2, 3 and 9 comprises four sieve stacks 1. Two stack frames 4, 5 are disposed symmetrically about a vertical axis 2 and a plane 3 containing the vertical axis. The stack frames 4, 5 comprise a solid bottom portion 6, 6, the delivery apertures not being shown in FIG. 9.

Each stack frame 4, 5 also comprises a top frame 7, 7', two outer solid struts 8, 8' and one tie rod 9, 9 in the center of two stack frames, when two or more sieve stacks are provided to each stack frame.

In FIGS. 1, 2 and 3, the vibrator or vibration exciter 12 is disposed between the two stack frames 4 and 5 in a drive casing 10 and is accessible through doors 11.

The vitrator 12 illustrated in the form of an unbalanced mass is rotatably mounted in a top bearing 13 and a bottom bearing 14 with the vertical axis 2 as the axis of rotation.

The unbalanced mass 12 comprises at the top an extended shaft end 15, which extends beyond the drive casing 10 and comprises a drive wheel 16. A drive motor 17 is mounted in the drive casing 10 with its axis parallel to the vertical axis 2. A pulley 18 fixedly keyed on the shaft of the drive motor 17 drives the unbalanced mass 12 through a belt 19.

The whole plansifter is mounted in freely vibrating manner by means of a vibratory mounting 20 consisting of four vibration bars A flexible electric cable 21 extends parallel to one of the vibration bars 20' to enable the current to be supplied to the drive motor 17. The drive casing 10 is fixedly connected, through distance rings 25 (not shown) and a screw connection 26, to the corresponding bottom and top elements of the stack frames 4, 5.

A safety device 27 is provided at the two outer corners of the sieve stacks l on the bottom portion 6. A holder 28 is fixedly connected to the bottom portion 6 and an angle piece 30 is pressed against the lowermost sieve frame of the sieve stack 1 by means of a screw 29 adjustable in the holder.

The sieve stacks I are formed of a large number of sieve frames 32. There is shown in plan view in FIG. 6 a sieve frame which comprises in known manner at the center the sieve portion 33 and at the periphery channels 34 adapted to the requirements. The sieve frame 32 is thrust against two abutment members 35 having lateral guide portions or lugs 36 extending over the entire height of the stack. As is shown in FIG. 7, the sieve frames 32 have a lug-like projection 38 at the bottom four corners, so that the sieve frames are mutually held and guided by the interengagement. Provided on the bottom elements are angular safety devices 27 which press the lowermost sieve frame and, due to the guiding of the sieve frames by means of lug-like projections 38, the whole sieve stack against the bearing members 35.

However, the actual transmission of the vibratory forces takes place by frictional engagement. The necessary frictional force is produced between the clamping device 41 movably connected to the top frame '7, and the corresponding counter-pressure applied by the bottom portion 6, which is rigidly connected to the top frame 7 through vertical elements 8, 9. The clamping device 41 comprises a clamping rod 42, which is provided with right-hand and left-hand threads 43 and 44 respectively. The clamping rod 42 has a bearing point 45 between the right-hand and left-hand threads 43 and 44 respectively, and on the outside a bearing 46. The bearings 45 and 46 are fixedly connected to the top frame 7. Clamping blocks 47 and 48 are disposed in the manner of nuts on the right-hand and left-hand threads 43 and 44 respectively, but are prevented from rotating by corresponding guides 49 on the top frame 7. When the clamping rod 42 is turned by means of a key 50, the clamping blocks 47 and 48 perform movements toward and away from one another respectively in a direction parallel to the axis of the clamping rod. The clamping blocks 47 and 48 each comprise a clamping lug 51 sliding in a corresponding guide slot 52 in the actual clamping frame 53. Due to the inclined arrangement of the guide slots 52, the whole sieve stack can be firmly pressed together by the clamping frame 53 by way of the crank 50 and the clamping rod 42. The symmetrical arrangement of two guide slots 52 in relation to the central axis 54 of the stack frames 4 and 5 respectively results in only small mechanical loading of the stack frames 4 and 5 respectively.

For each sieve stack, there is provided in FIGS. l, 2 and 3 at least one material inlet duct 60, as well as two or more delivery ducts 61.

The actual sifting operation will not be dealt with, since it is known.

For the assembly of the sieve frames and for setting in operation the plansifter, the following procedure is adopted:

The individual sieve frames 32 are first stacked on the bottom portion 6 and are thrust against the abutments 35. The sieve frame bearing on the bottom portion 6 is pressed against the abutments by two safety devices 27 disposed at the corners, and the clamping device 41 is lowered. The whole sieve stack is so secured as to prevent dislodgment of individual sieve frames by means oflug-like projections on the latter. by means of the safety device 27 and by'means of two guiding angle pieces 55 disposed on the clamping frame 53, the clamping device 41 merely being lowered on to the sieve stack. This prevents any possible accident or destruction of the sieve frames in the event of the vibrator 12 being unexpectedly set in operation before the clamping device 41 has been completely tightened. However, lengthy operation with the clamping device 41 in the slackened condition must be avoided. The attention of the operating personnel is rapidly drawn to this error by the noise which may be set up by the loose sieve frames 32, and the error can be rectified by firmly tightening the clamping rod 42 by means of the key 50.

The plansifter according to the invention does in fact afford the advantages of the known closed large-scale plansifters, as also the advantages of open small-scale plansifters.

In the known large plansifters, the whole plansifter is surrounded by a rigid and stable frame on a longitudinal plane. The frame, either horizontal or vertical, surrounds two or more sieve stacks and a drive casing situated therebetween. Consequently, the frame positively encloses the vibrator on the one hand and the whole vibratory mass of the plansifter on the other handin a vertical or horizontal plane. In such known plansifters, except those having a square base, the frames extend over the longitudinal plane of the plansifter by virtue of the basic principle.

For a readier explanation of the solution according to the invention, we define as the longitudinal plane (A) or (B) the vertical plane which intersects the sieve stacks and the drive or its casing.

We define as the transverse plane (C) the vertical plane extending perpendicularly to the longitudinal axis and intersecting only the sieve stacks.

In the plansifter according to the invention, the stack frame 4, 5 extends around only one or more sieve stacks I and lies transversely in relation to the position of the above-defined longitudinal frames in a vertical plane. The stack frame 4, 5 according to the invention is a vibrationally stable unit due to its rigid construction together with the clamping device 41 and one or more sieve stacks l. The forces are guided in this vibrationally stable unit in top and bottom elements respectively. The sieve stack is clamped to the bottom portion 6 as by the action of a necked-down screw. The vibration-exciting forces and the mass forces of the sieve stack are transmitted, except in the case of the top and bottom sieve frames, only through frictional forces to the bottom portion 6 and through the clamping frame 41 to the top frame 7. However, the vertical guiding of the individual sieve frames can be retained for safety reasons. Enclosure parts or even doors may be omitted for the purpose of sieve mounting. The clamping force of the clamping device 41 is a multiple of the mass forces of the sieve frames themselves. In absolute terms, therefore. the vertical forces in the central plane 54 of a stack frame are greatest, which pass through correspondingly dimensioned vertical elements 8, or where a plurality of sieve stacks is provided, through additional vertical elements 9. This new concept of a complete frame closed in the flow of forces in the transverse plane makes it possible completely to achieve the underlying object of the invention in the double arrangement according to the invention.

The stack frame may contain in the vertical direction as many sieve frames 32 as there are in the known closed large plansifters. In a plansifter according to the invention, two, four, six or even more stacks can be readily accommodated, that is to say, this concept is at least equivalent to the known large plansifters in regard to the forces produced and above all, of course, in regard to the cost of manufacture. After slackening of the clamping rod 42, the individual sieve frames can be individually removed, that is to say, access may be had to them substantially at any time. Due to the fact that the forces are taken up in an open frame construction, a continuous visual observation is possible on at least three sides of the stacks in the construction according to the invention. More particularly in the chemical industry, in which losses of material being sifted must be immediately noticed in many cases, the plansifter according to the invention affords a considerable advance. In addition, it is desirable precisely in these branches of industry for the whole plansifter, i.e. the sieve stacks themselves, to be cleaned with water or another cleaning agent which is applied thereby by spraying or blowing, and the construction according to the invention is very particularly suitable for this purpose.

In a particularly advantageous plansifter, the drive casing comprises vibrationally stable top and bottom connecting elements 22 and 23 respectively, as shown in hatched form in FIG. 9, and the vertical elements of the stack frames form the vertical edge of a rigid cubelike vibrating frame.

A surprising factor of this embodiment of the invention is the ideal form of the cube-like frame as obtained, which can be approximated or obtained in all cases by appropriate variations of the distance between the stack frames. Nevertheless, the system of unit construction can still be followed. Each of the two stack frames with the associated sieve stacks forms a transportable and mountable unit. The top and bottom connecting elements 22, 23 forming the drive casing afford, with small auxiliary struts at thefour corners with an installed vibrator, a transportable unit.

In the construction according to the invention, the

horizontal vibratory forces are directed into corresponding horizontal bottom and top frames respectively. The mass forces are also introduced into this plane by frictional forces, whereby an equilibrium of forces is obtained.

The center of gravity of the vibrator and of the plansifter are brought to the same horizontal plane both in the empty state and in the ideally loaded state, i.e. during sifting of product. This is achieved by balancing at the vibrator, so that the oscillation pattern is the same over the entire height. With considerable load fluctuations, displacements of the center of gravity of the plansifter occur. The displacement of the center of gravity in the plansifter is very minute, since the relatively large mass of the plansifter is offset by only a small mass of the variable product. The ratio of variable product mass to plansifter mass is about 1:20.

Considerable resistance is set up to the resultant relatively small displacement forces acting on the top frames 7, 7 in relation to the bottom portion 6, 6 due to the stable cube form with the aid of the compressed sieve stacks 1.

In a further advantageous plansifter, the vibrationexciting forces are passed directly into the top and bottom elements of the stack frames through a few punctiform connecting elements 25, 26. For a smaller plansifter, for example, four screw connections 25, 26 per stack frame are sufficient, that is to say only eight screws are required for the whole plansifter. This renders possible a very rapid assembly of the three elements. The flow of forces is consequently accurately controlled, and rusting at the fitting parts, with the resultant detrimental effects on screws and frames, is avoided.

In a further plansifter according to the invention, the

. vertical elements 8, 9 of the stack frames 4, 5 are disposed on the central plane 54 of the stack frame, containing the center of gravity of the stack. In this way, a uniform distribution of stresses in the frame is obtained.

In a further very advantageous plansifter according to the invention, the vibratory mountings 20 aredisposed on the four vertical elements 8 of the stack frame 4, 5. Particularly in the case where the ceiling height is low, the mounting may be disposed at any level, and if necessary even at the level of the bottom frame 6.

The fact that the end position of the clamping devices is in the neighborhood of the vertical central plane 54 of the stack frames 4, 5 is very advantageous to the plansifter according to the invention. The clamping device 41 thus transmits the force almost only in the form of a compressive force without large bending moments on the corresponding top frame 7, 7'.

In a further form of construction of the plansifter according to the invention, each clamping device 41 is individually positively guided in a slot S in the top element 7, 7' and has at least one angular abutment 55.

In order that the stacks of sieves can be arranged symmetrically with respect to the vibration generating means, which for instance is an unbalanced weight rotating about an axis, each rigid frame of the plurality of rigid frames is arranged laterally of the vibrationgenerating means. The words transverse, the transverse direction etc., in this specification and claims refer to a direction which is transverse to a vertical plane passing centrally through both the given frame and the vibration-generating means. Thus for instance, where there are two frames arranged with the vibration generating means in between them, the frames may be considered as being at longitudinally opposite ends of the plansifter and the transverse direction for each frame is the direction transverse to that longitudinal direction of the plansifter.

What is claimed is: 1. In a plansifter having a vibratory mounting and at least two sieve stacks each formed by a number of sieve frames mounted one upon the other and held together by stack clamping devices which produce vertical forces, said stacks of sieves being disposed in symmetrical positions about a vibrator, the improvement comprising two upright stack frames, each of which embraces as a unit at least one of said sieve stacks, said stack frames each having a top element and a bottom element connected together and to a vibration generator so that the forces of the vibration generator are passed into said sieve stacks through said top and bottom elements, said top and bottom elements being rigidly closed in themselves in the transverse direction, said clamping devices cooperating with said top and bottom elements to hold said sieve stacks therebetween.

2. A plansifter according to claim 1 wherein a drive casing containing said vibration generator is disposed between said stack frames, said drive casing comprising vibrationally stable top and bottom connecting elements which are connected to said top and bottom elements of said stack frames.

3. A plansifter according to claim 2 wherein the vibration-exciting forces from the drive casing are directed through puncti-form connections immediately into the top and bottom elements of the stack frames.

4. A plansifter according to claim 2 wherein the top and bottom elements of each of the stack frames are connected by vertical elements spaced from the drive casing and disposed in a central plane bisecting the stack vertically and containing the center of gravity of the stack.

5. A plansifter according to claim 4 wherein vibratory mountings for the plansifter are disposed on the vertical elements.

6. A plansifter according to claim 5 wherein said clamping devices include a movable clamping frame in contact with each said stack and movable clamping blocks mounted between the clamping frame and the stack frame to produce said vertical forces, and wherein for the end positions of the clamping frames in their clamping condition, the clamping blocks lie in the neighborhood of said vertical central plane.

7. A plansifter according to claim 6 wherein the clamping devices are positively guided in the top element of each stack frame and each comprises at least 10 one guiding angle piece to guide and initially hold the top sieve frame in position.

8. A plansifter according to claim 2 wherein two sieve stacks are mounted on each stack frame, and wherein there are provided for each stack, on the stack side closer to the other stack on the same frame, abutment members having short lateral guide lugs extending over the entire height ofthe stack.

9. A plansifter according to claim 8 wherein, on the bottom element of each stack frame, clampable safety angle members are provided at two corners of each stack, opposite said abutment members.

10. In a plansifter having a vibratory mounting and at least two sieve stacks each formed by a number of sieve frames mounted one upon the other and held together by stack clamping devices which produce vertical forces, said stacks of sieves being disposed in symmetrical positions about a vibrator, the improvement comprising two upright stack frames, each of which embraces as a unit at least one of said sieve stacks, said stack frames each having a top element and a bottom element connected together and to a vibration generator so that the forces of the vibration generator are passed into said sieve stacks through said top and bottom elements, said top and bottom elements being rigidly and statically closed in themselves in the transverse direction, said clamping devices cooperating with said top and bottom elements to hold said sieve stacks therebetween, said top elements and said bottom elements being respectively connected together through vibrationally stable top and bottom intermediate members, said top and bottom elements of each stack frame being connected together by a pair of oppositely disposed vertical elements forming the corners of a cubelike vibrating frame.

11. A plansifter according to claim 10 wherein a drive casing containing said vibration generator is disposed between said stack frames, said drive casing comprising vibrationally stable top and bottom connecting elements which are connected to said top and bottom elements of said stack frames.

l= l =l 

1. In a plansifter having a vibratory mounting and at least two sieve stacks each formed by a number of sieve frames mounted one upon the other and held together by stack clamping devices which produce vertical forces, said stacks of sieves being disposed in symmetrical positions about a vibrator, the improvement comprising two upright stack fRames, each of which embraces as a unit at least one of said sieve stacks, said stack frames each having a top element and a bottom element connected together and to a vibration generator so that the forces of the vibration generator are passed into said sieve stacks through said top and bottom elements, said top and bottom elements being rigidly closed in themselves in the transverse direction, said clamping devices cooperating with said top and bottom elements to hold said sieve stacks therebetween.
 2. A plansifter according to claim 1 wherein a drive casing containing said vibration generator is disposed between said stack frames, said drive casing comprising vibrationally stable top and bottom connecting elements which are connected to said top and bottom elements of said stack frames.
 3. A plansifter according to claim 2 wherein the vibration-exciting forces from the drive casing are directed through puncti-form connections immediately into the top and bottom elements of the stack frames.
 4. A plansifter according to claim 2 wherein the top and bottom elements of each of the stack frames are connected by vertical elements spaced from the drive casing and disposed in a central plane bisecting the stack vertically and containing the center of gravity of the stack.
 5. A plansifter according to claim 4 wherein vibratory mountings for the plansifter are disposed on the vertical elements.
 6. A plansifter according to claim 5 wherein said clamping devices include a movable clamping frame in contact with each said stack and movable clamping blocks mounted between the clamping frame and the stack frame to produce said vertical forces, and wherein for the end positions of the clamping frames in their clamping condition, the clamping blocks lie in the neighborhood of said vertical central plane.
 7. A plansifter according to claim 6 wherein the clamping devices are positively guided in the top element of each stack frame and each comprises at least one guiding angle piece to guide and initially hold the top sieve frame in position.
 8. A plansifter according to claim 2 wherein two sieve stacks are mounted on each stack frame, and wherein there are provided for each stack, on the stack side closer to the other stack on the same frame, abutment members having short lateral guide lugs extending over the entire height of the stack.
 9. A plansifter according to claim 8 wherein, on the bottom element of each stack frame, clampable safety angle members are provided at two corners of each stack, opposite said abutment members.
 10. In a plansifter having a vibratory mounting and at least two sieve stacks each formed by a number of sieve frames mounted one upon the other and held together by stack clamping devices which produce vertical forces, said stacks of sieves being disposed in symmetrical positions about a vibrator, the improvement comprising two upright stack frames, each of which embraces as a unit at least one of said sieve stacks, said stack frames each having a top element and a bottom element connected together and to a vibration generator so that the forces of the vibration generator are passed into said sieve stacks through said top and bottom elements, said top and bottom elements being rigidly and statically closed in themselves in the transverse direction, said clamping devices cooperating with said top and bottom elements to hold said sieve stacks therebetween, said top elements and said bottom elements being respectively connected together through vibrationally stable top and bottom intermediate members, said top and bottom elements of each stack frame being connected together by a pair of oppositely disposed vertical elements forming the corners of a cube-like vibrating frame.
 11. A plansifter according to claim 10 wherein a drive casing containing said vibration generator is disposed between said stack frames, said drive casing comprising vibrationally stable top and bottom connecting elements which are connected to said top and bottom elements oF said stack frames. 